The compound 2,2-bis(4-hydroxyphenyl)propane, also known as bisphenol-A, enjoys major use as a co-reactant for phosgene in the production of aromatic polycarbonate resins suitable for molding applications. It is known that 2,2-bis(4-hydroxyphenyl)propane can be prepared by the acid-catalyzed reaction of acetone with a substantial excess of phenol. Upon completion of the reaction and after the removal of the acid catalyst and by-product water, a mixture comprising bisphenol-A, unreacted phenol and organic by-products of the condensation reaction results.
Various methods for recovering the bisphenol-A from its reaction mixture have been proposed in the art. Dugan et al, U.S. Pat. No. 3,326,986, discloses a procedure wherein "crude" bisphenol-A is separated from its reaction mixture by precipitation or distillation, the "crude" bisphenol-A is melted in the presence of water, and the melt is thereafter crystallized to obtain the bisphenol-A in purer form.
Luten, U.S. Pat. No. 2,791,616, discloses a process for recovering bisphenol-A from a phenol-rich reaction mixture wherein the bisphenol-A and phenol are separated in equimolar proportions in the form of a crystalline adduct, the adduct is thereafter treated to selectively melt only the phenol portion, and the bisphenol-A remaining in solid crystalline form is collected.
Also of interest are Keller et al, U.S. Pat. No. 2,777,183, who uses an alkaline substance to partially neutralize the bisphenol compound during purification; French Pat. No. 1,580,676, which discloses purifying bisphenols by repeated washings with warm water; Boroviska et al, Chem, Abstracts 60:2832(c), which uses even hotter water to purify bisphenols; German Pat. No. 971,013 (1954), which also involves neutralization with base in the purification of bisphenols; Czech Pat. No. 147,105 (1973), which describes countercurrent extraction processes for the purification of bisphenols; Japanese Pat. No. 30269/73, which uses an emulsion of an organic solvent, surfactant and water to purify bisphenol; Japanese Patent Publication No. SHO-45-39251 which discloses distilling an adduct of bisphenol and phenol in the presence of a high melting glycol; Japanese Patent Publication No. SHO-45-22539 which discloses distilling an adduct of bisphenol and phenol in the presence of an aliphatic dicarboxylic acid ester; DeJong, U.S. Pat. No. 3,535,389, who mixes the impure bisphenol with a water-organic solvent mixture and steams out the solvent; Polish Pat. No. 57,925 describes melting bisphenols in water, acid and salt, and then crystallizing; Levkovich et al, Chemical Abstracts Vol. 55:11370h who free bisphenols of impurities by washing with dilute ammonium hydroxide; and Marx et al, U.S. Pat. No. 3,162,690 who form adducts of bisphenols with cresols, then decompose them for purification.
There has now been discovered a new and very simple process for recovering bisphenol-A from a reaction mixture resulting from the condensation of acetone and excess phenol, wherein the bisphenol-A is cleanly separated from a melt of both the phenol and the bisphenol-A in the form of highly desirable, easy-to-handle rhombic crystals, without the need to use high temperatures, liquid-liquid extraction, extraction with water containing surfactants, or sodium chloride, or acid, or alkali or ammonium hydroxide, etc.